The invention relates to a device for injecting a fluid, for example a fuel, in particular for an internal combustion engine.
More precisely, the invention relates, according to a first of its aspects, to a fluid injection device having a main injection axis and comprising:                a nozzle comprising, on said axis, an injection orifice and a seat and being, at the opposite end, connected to a casing,        a needle having, on said axis, a first end defining a valve element, in a zone of contact with the seat and being, at the opposite end, connected to an actuator mounted so as to be able to move axially in the casing in order to vibrate the needle, providing between its first end and the seat of the nozzle a relative movement capable of alternately opening and closing the valve element, the actuator comprising, on the axis, a first portion, a second portion and a third portion suitable for being traversed by acoustic waves initiated by vibrations of the second portion, the first portion and third portion being placed axially on either side of the second portion, which comprises an electroactive material, the three portions being squeezed together in order to form a block having axially two opposite limits, the first portion being connected to the needle at the location of one of said limits,        excitation means for vibrating the second portion of the actuator with a setpoint period τ.        
Such an injection device, called an injector, makes it possible to obtain a cyclic opening with the setpoint period τ, at a controlled frequency that is for example ultrasonic and at a controlled amplitude, of the valve element of the injector, in particular during an established speed of its operation, that is to say during operation at a predetermined temperature outside the starting and stopping phases of the injector. A layer formed by the fluid escaping from the nozzle at the opening of the valve element is broken up and forms fine droplets. In an application of the injector in which it sprays fuel into a combustion chamber, the fine droplets promote a more homogeneous air-fuel mixture, which makes the engine less polluting and more economical.
According to known devices, the cyclic opening of the valve element is carried out with the aid of conventional vibration means, for example piezoelectric and/or magnetostrictive means with corresponding excitation means. The vibration means are arranged in the actuator having axially two opposite limits, one of which, called the first limit, is connected to the needle. Excited by the vibration means, the actuator converts an electric energy into vibrations of its first limit, with the setpoint period τ and a predetermined amplitude. The actuator acting, via its first limit, directly on the needle therefore plays a role of an active “master” controlling the needle which is then a passive controlled “slave”. Specifically, the vibrations of the first limit of the “master” actuator produce longitudinal alternating movements of the “slave” needle and therefore of its first end, relative to the seat of the nozzle. In order to provide a sufficient flow rate of fuel when the valve element opens, it is necessary for the head of the needle and the nozzle to be made to resonate substantially in phase opposition. For this, the characteristic lengths of the needle and that of the nozzle are chosen, in a known manner, so that the acoustic wave propagation times in respective materials forming the needle and the nozzle are equal to a quarter of the period of the vibrations τ/4 or to odd multiples of said quarter of the period, that is to say equal to [2j+1]*τ/4 with a positive, non-zero integer multiplying coefficient j. Resonating “needle/nozzle” and “needle/actuator” structures thus formed generate high amplitudes of opening of the valve element at low pressures, for example, equal to or less than 5 MPa, in the combustion chamber. Gradually, as the fuel is injected during a compression cycle the pressure in the combustion chamber and, consequently, a backpressure at the valve element increases. This backpressure may also vary according to the point of operation of the engine. With the increase in the backpressure, the intensity of the impacts of the first end of the needle on its seat, even damped by the layer of fuel, becomes ever greater. The feedback of these impacts, on the one hand, in the resonating “needle/nozzle” structure as a conventional quarter wavelength [2n+1]*τ/4 and, on the other hand, in the other resonating “needle/actuator” structure induces a coupling between the impact and a lifting of the first end of the needle from its seat by modifying the amplitude of opening of the valve element. If the impacts persist, the lifting of the head becomes chaotic. The benefit of the resonances is lost. The opening of the valve element becomes chaotic thus reflecting a disordered operation of the injector which may render the flow rate of fuel difficult to control.